The present invention relates to the field of signal analysis, and more particularly to the display of signal characteristics in a single three-dimensional format.
Various instruments can monitor and analyze an input signal and display the characteristics of the input signal in a two-dimensional format, such as frequency versus time or power versus time. Further the amplitude may be indicated in terms of a maximum and minimum value during a particular time interval or a particular frequency range. For some signals more than two characteristics are needed to adequately describe the signal to an engineer. This requires displaying either a pair of two-dimensional displays simultaneously having different axes, or, as in the 3052 Spectrum Analyzer manufactured by Tektronix, Inc. of Beaverton, Oreg., displaying a succession of two-dimensional frequency versus amplitude displays in an historical display, such as a waterfall display.
The collection of data that includes three characteristics of the input signal, such as amplitude and frequency over time, provides an abbreviated frequency/amplitude history of the signal monitored. Such a history is a valuable aid to an engineer for detecting frequency and amplitude modulation, either intended or unintended, in a signal. A history is most valuable, however, if it is displayed graphically in a manner that allows an engineer to completely and efficiently detect the modulation. Also, if a history can be compactly represented, a number of histories can be placed on a single output device--be it a graphics CRT display, a printer or plotter, or any other device capable of displaying graphics--for comparison or for simultaneous inspection.
The 5371A Frequency and Time Interval Analyzer, manufactured by Hewlett-Packard Co. (HP), of Palo Alto, Calif., monitors and analyzes an input signal, compiling a collection of average frequency data. This product is described in more detail in "Product Note/Specification Guide--HP 5371A Frequency and Time Interval Analyzer", (Hewlett--Packard, 1988), which is hereby incorporated by reference. As with the Tektronix 3052 Spectrum Analyzer, each average frequency datum in the collection is the average frequency of the input signal during a separate sampling interval. The HP 5371A can measure the maximum amplitude of the input signal, but the measurement is free-running and therefore is not associated with a particular sampling interval.
The HP 5371A Frequency and Time Interval Analyzer does provide a method for displaying the signal characteristics that it can measure. For displaying average frequency, two methods are provided. The first is a graph of the time variation of frequency. The second method is a histogram, where each bin of the histogram represents a frequency range and the height of a rectangle within a bin represents the number of sampling intervals in which the average frequency of the signal falls within the range. Maximum signal amplitude, though of limited value for analyzing amplitude modulated signals, may be also graphed versus time.
Using the time variation graphs of the HP 5371A, it is possible to display the signal characteristics--average frequency, maximum amplitude, and minimum amplitude. Superimposing graphs of maximum amplitude versus time and minimum amplitude versus time onto a graph of average frequency versus time would allow an engineer to completely characterize the frequency and amplitude modulation behavior of a signal under analysis. However, this method would force an engineer to integrate data from several locations on a display and, furthermore, would not result in the compact representation desired.
Some instruments provide quasi-3-dimensional displays, as discussed above for the 3052 waterfall display. A waterfall display contains a number of planar amplitude versus frequency graphs, the graphs representing the frequency spectra of an input signal at different times. The graphs are stacked closely together, each amplitude versus frequency graph displaced upward and to the right relative to the newer one "in front of" it. Thus, older graphs appear to recede into the display. However, a frequency/amplitude versus time characterization determines a line, and the line lacks shape. Displacement of a line will not create the appearance of the line receding into the display.
Therefore, what is desired is a method of simultaneously displaying, in a compact representation, a collection of data characteristics representing the frequency and amplitude histories of a signal.